In many systems using fluids it is important to know the level of non-soluble contamination in the fluid, either because the contaminant will damage the system and/or because the presence of contaminant indicates wear or breakdown in the system. In other fluids it is desirable to maintain the level of particulate material therein at an approved or known value. Present methods of automatically monitoring the level of contaminant involve expensive and complicated equipment and in most cases the tests can only be performed by highly trained personnel. It is an object of this invention to provide an apparatus which can give an indication of such contamination in a simple and speedy manner.